A method of this type is used for example in the production of substrateless luminescence diodes based on GaN. Such components contain a semiconductor body and a carrier part, on which the semiconductor body is fixed. In order to produce the semiconductor body, firstly a semiconductor layer is fabricated on a suitable substrate, subsequently connected to a carrier and then stripped away from the substrate. Dividing up, for example sawing up, the carrier with the semiconductor layer arranged thereon produces a plurality of semiconductor bodies which are in each case fixed on the corresponding carrier part. What is essential in this case is that the substrate used for producing the semiconductor layer is removed from the semiconductor layer and does not simultaneously serve as a carrier or carrier part in the component.
This production method has the advantage that different materials are used for the substrate and the carrier. The respective materials can thus be adapted, largely independently of one another, to the various requirements for the production of the semiconductor layer, on the one hand, and the operating conditions, on the other hand. Thus, the carrier can be chosen in accordance with its mechanical, thermal and optical properties independently of the requirements made of the substrate for the fabrication of the semiconductor layer.
The epitaxial production of a semiconductor layer, in particular, makes numerous special requirements of the epitaxial substrate. By way of example, the lattice constants of the substrate and of the semiconductor layer to be applied have to be adapted to one another. Furthermore, the substrate should withstand the epitaxy conditions, in particular temperatures of up to in excess of 1000° C., and be suitable for the epitaxial accretion and growth of an as far as possible homogeneous layer of the relevant semiconductor material.
By contrast, other properties of the carrier such as, by way of example, electrical and thermal conductivity and also radiation transmissivity in the case of opto-electronic components come to the fore for the further processing of the semiconductor body and operation. Therefore, the materials suitable for an epitaxial substrate are often only suitable to a limited extent as carrier part in the component. Finally, it is desirable, particularly in the case of comparatively expensive epitaxial substrates such as silicon carbide substrates, for example, to be able to use the substrates repeatedly.
The stripping-away of the semiconductor layer from the substrate is essential for the aforementioned production method. Said stripping-away can be achieved by irradiating the semiconductor-substrate interface with laser radiation. In this case, the laser radiation is absorbed in the vicinity of the interface, where it effects decomposition of the semiconductor material.
The semiconductor layer may be separated from the substrate for example by means of laser stripping, as described in U.S. Pat. No. 6,559,075. In this case, the frequency-tripled radiation of a Q-switch Nd:YAG laser at 355 nm is used for stripping GaN and GaInN layers from a sapphire substrate. The sapphire substrate is transparent to radiation at this wavelength. The radiation energy is absorbed in a boundary layer having a thickness of approximately 100 nm at the junction between the sapphire substrate and the GaN semiconductor layer. At pulse energies of above 200 mJ/cm2, temperatures of more than 850° C. are reached at the interface. The GaN boundary layer decomposes at this temperature to liberate nitrogen, and the bond between the semiconductor layer and the substrate is separated.
In the case of a method of this type, there is the risk of substrate residues adhering on the semiconductor layer on account of incomplete material decomposition during the stripping away of the semiconductor layer. By way of example, microscopic sapphire grains, so-called “flakes”, are often found on a GaN layer separated from a sapphire substrate in this way.
The diameter of these sapphire residues typically lies between 5 μm and 100 μm. The sapphire residues make further processing of the semiconductor layer more difficult and require a comparatively high effort to remove them on account of the high mechanical and chemical resistance of sapphire. This may have the effect that only parts of the stripped-away semiconductor layer can continue to be used or the entire layer even becomes unusable.
Generally, a mechanical stabilization of the semiconductor layer to be stripped away is necessary since the layer thickness is so small that otherwise there is the risk of damage, in particular a break or crack in the layer. By way of example, a connection of the semiconductor layer, which may also already be partly processed, to the carrier by means of a material joint is suitable for the purpose of mechanical stabilization. Such a connection should be thermostable at least to an extent such that it withstands without damage the temperatures that occur during subsequent fabrication steps. Furthermore, said connection should also remain stable in the event of alternating temperature loads which may occur, in particular, during operation of the component.
Adhesives are often used for fixing the semiconductor layer on the carrier. In the case of relatively high electrical powers, problems may result in this case on account of the limited thermal and electrical conductivity of adhesives. The limited thermal endurance of such adhesive connections additionally limits the permissible temperature range of a corresponding component and consequently the maximum possible power loss.